1. Field of the Invention
This invention relates generally to housings and supports for optical elements and the like and, more particularly, to an improved kinematic mounting and housing arrangement for optical components having improved stress and strain compensation characteristics.
2. Description of the Prior Art
In many optical applications it is desirable to prevent any movement of certain optical elements when exposed to thermal expansion of member or members which support the optical elements. For example, a lens, mirror, or prism mounted in a close fitting metal cell could be constructed and designed to maintain perfect relative alignment with the cell in the absence of ambient temperature variations. However, under certain varying operating conditions, such as a decrease in temperature, the metal cell, because its coefficient of thermal expansion is normally higher than that of glass, will contract more rapidly than the glass, and may cause the glass to fracture or at least be stressed to the point of distorting the optical image which is transmitted through the glass or reflected therefrom. Conversely, as the temperature increases, the cell will expand more rapidly than the glass, leaving the lens unrestrained and free to move laterally within the cell, causing the optical element to become out-of-focus.
Various means and methods have been used and proposed in an attempt to avoid damage to an optical element, such as a lens or prism or IR sensor, while still restraining the element sufficiently to prevent excessive lateral displacement of the optical element within its cell. Such displacement ordinarily results when the combined lens and mount are subjected to wide temperature variations. A common method for mounting an optical element in a metal cell is to encircle the element with a thin endless metallic strip which is pressed into place around the element and which is attached to the cell. However, in applications where wide temperature variations are encountered, this method is objectionable because an excessive radial load must be applied by the strip to the element in order to retain the holding power of the strip. Also, extraordinary dimensional accuracy of the metallic ring and optical element is necessary in order to properly fit the ring into the optical element and, in practice, it is usually necessary to grind the element to fit the encircling ring in order to obtain the proper fit. This is costly and time consuming.
Another commonly used mounting method for fixedly supporting optical elements involves the use of spring loaded members equally spaced around the periphery of the element and positioned between the element and its supporting cell. A disadvantage of this method is the difficulty of positively aligning the element in the cell. Another disadvantage of this latter method is that the spring constants of the loading springs are or become not equal, especially after being subjected to a series of wide temperature variations. This generally results in relative misalignment of the elements after extended use of the instrument in which the optical elements are mounted.
One of the most common methods of fixedly mounting optical elements is the interposition of some resilient material between a lens and its retaining cell for the purpose of absorbing lateral shock, and compensating for the effects of temperature variation and attendant thermal expansion of the lens and cell. The resilient material usually takes the form of a gasket or a series of equally spaced shims for effectively centering the element. One major objection to this method is that the interposed material is subject to deterioration and loss of resiliency after the passage of time or after the optical system for the above hardware has been subjected to a series of wide temperature variations. Such deterioration of the interposed material may result in loss of relative alignment of the optical axis of the various optical elements in the system.
A method which has been used to overcome some of the disadvantages of the last-described method involves the use of tangent straps which are placed around the optical element. These straps provide relatively low elastic joints which are useful to minimize thermal expansion strains while allowing the optical element to remain fixed. The major objection to this method is that the thermal expansion strains are not entirely relieved due to the low elasticity of the tangent straps, causing the transmission through the optical element to become distorted. Additionally, the straps do not compensate for loading that causes the optical element to move out-of-plane.